This invention concerns allocation of radio channels in a radio system using both time division and frequency division at the same time.
In a radio system, the great number of mobile terminal equipment, such as mobile stations, on the one hand and the band width requirements of multimedia applications on the other hand make it necessary to utilize as efficiently as possible the frequency spectrum allocated for the system. The interference caused by the radio environment or by other terminal equipment of the system is a fundamental factor limiting reuse of frequencies. It can be reduced by developing efficient radio subsystems and channel allocation techniques. Time and space diversity, low-noise filters and efficient channel correctors and modes of modulation are used in radio and transfer subsystems to reduce interference and to separate the desired signal. However, the factor limiting system total capacity more than any other factor is co-channel interference caused by reuse of frequencies. In fact, the basis for algorithms used in channel allocation is to use loss characteristics of radio wave propagation to minimize the carrier-to-interference or CIR ratio and thus to boost reuse of the radio spectrum.
Channel allocation means dividing a given frequency spectrum into a set of adjacent or non-interferring channels, all of which may be used simultaneously while the quality of the received signal remains acceptable. Several known techniques are available for dividing channels, such as Frequency Division (FD), Time Division (TD) or Code Division (CD). In frequency division, the separation of channels is brought about by dividing the spectrum into adjacent frequency bands, in time division by dividing the frequency into sequential time periods or time slots, while in code division the separation of channels is obtained by using different modulation codes. As an example of a cellular system using a combination of FD and TD, the known GSM mobile system is mentioned, whereas Digital European Cordless Telecommunication (DECT) is mentioned as an example of a TD system.
The distance at which the same channel may be reused so that CIR remains acceptable, is called the reuse distance of the same channel. On the other hand, since CIR is a function of the reuse distance and transmission power, the interference level may be lowered in a cellular system by increasing the distance between base stations or by using dynamic control of the transmission power.
At least fixed channel allocation (FCA) and dynamic channel allocation (DCA) are channel allocation methods.
In fixed channel allocation, the range is divided into cells, for each of which a set of frequencies is allocated according to some reuse pattern. Simplicity is an advantage of this allocation, but it suffers from an inability to adapt to traffic situations and to the numbers of mobile subscribers in the cells.
In dynamic channel allocation, all channels are in a xe2x80x9cchannel poolxe2x80x9d and channels are taken from the pool for new calls as calls come to the system, making sure at the same time that the minimum CIR is maintained. Thus, there is no fixed relation between channels and cells, but any channel may be choosed by any cell provided that the signal interference is acceptably low. Different dynamic ways of allocation can be distinguished depending on how the channels are chosen. Advantages of this allocation are great flexibility and an ability to adapt to changing traffic, but it is less efficient than fixed channel allocation if the load is very high. Dynamic channel allocation is described thoroughly in the publication IEEE Personal Communications, June 1996; Channel Assignment Schemes for Cellular Mobile Telecommunication Systems; A Comprehensive Survey; I. Katzela, M. Naghshineh.
The present TDMA/FDD-based cellular mobile systems mainly use fixed channel allocation FCA. Advanced systems also use frequency jumping to improve the quality of the connection for averaging loss and interference. In a frequency jumping pattern only the traffic channel frequency can be changed or, besides that, also the time slot. Should the signal quality become poorer and fall below an acceptable limit due to a change in the environment or a movement of the mobile, a change of channel or handover is performed, whereby the channel is exchanged for another while the connection remains the same with the base station (a so-called intra cell handover) or the connection is transferred to pass through another base station (so-called intra cell handover). Handover may thus be caused by qualitative deterioration of the radio link, or then the network may command the mobile to perform the handover, because the network wishes to rearrange the channels in order to avoid congestions.
In the first-mentioned case, the mobile performs continuous measurements of the connection quality, usually calculation of the bit error ratio, and measurements of carrier powers of adjacent base stations and transmits the measurement results to the network at regular intervals. The network uses these measurement results in its making of decisions on handover. In such a case, the handover is mobile assisted handover. If in intracellular handover no free channels are found from the cell""s own base station or in channel exchange between cells from that adjacent base station to the cell of which the mobile is going, then disconnection must be done. When the capacity is increased in a cellular system by reducing the cell size and in a system, where the cell size is very small from the start, in a so-called mikrocell system, channel exchange must be done several times during a call. In addition, when the number of users grows, not only will it be increasingly more difficult to maintain a good service level, but it will also become increasingly probable that no free channels will be found.
Dynamic channel allocation is used in the DECT system, which is in accordance with TDMAITDD, that is, transmission and reception take place at the same frequency. In such a system channel exchange may be performed by the mobile and not by the network as in, for example, the GSM system. This is possible because the mobile is able to monitor all channels in the uplink and downlink directions and thus it has constant information about the condition of the radio interface, that is, about free and busy channels and about their interference levels.
To increase the capacity in any TDMA/FDD system it would be natural to use dynamic channel allocation DCA instead of fixed channel allocation, but in such a way that the mobile and not the network could choose the channel from the set of free channels. Hereby the new channel would be taken from a pool containing several channels also in channel exchange situations. A problem when using DCA in this form is that although many channels are available, all of them can not be used. The mobile can not perform the choice of a new channel at random, because a channel picked at random may already be in use. Since the mobile lacks information about the state of the radio interface, especially about the state of channels in the uplink direction, the mobile is thus unable to perform any exchange of channels. No method of acquiring such information has been presented so far.
It is an objective of the present invention to provide a method allowing use of dynamic channel allocation also in TDMA/FDD systems so that a mobile is able itself to choose the channel it will use and thus to perform channel exchange. Channel exchange must be possible at least within the cell and at least to another time slot of the same frequency, but preferably also from one cell to another.
The established objective is achieved with the attributes described in the independent claims.
According to the invention, a special very low-level signal, which is called pilot signal hereinafter, is transmitted continuously on a carrier from a base station. This is done for every avilable carrier and in its every time slot irrespective of whether or not a useful signal (traffic bursts) is transmitted in the time slot. The information contained in the pilot signal is time slot synchronized so that the same information is transmitted in each time slot. Hereby, in order to obtain all information, it will suffice at a minimum that the receiver receives just any one time slot at the frequency in question and decodes the information of the pilot signal from it.
In the time slot where the useful signal is transmitted, the pilot signal level is lower than that of the useful signal proper and it has the same band width as a normal useful signal. However, it is coded in a different way than the useful signal, so even though the frequency is the same as for the useful signal, the pilot signal can be detected in the receiver.
In a time slot where the useful signal is not transmitted, the pilot signal level is very low, but still such that although it almost drowns in noise, it can still be extracted therefrom.
The pilot signal contains a limited number of information concerning the base station. The most important piece of information is whether the channels of the concerned frequency are available for dynamic channel allocation, that is, whether uplink direction channels corresponding to downlink directions are in use. When desired, the signal can also contain information identifying the base station, whereby based on this information the mobile is able to distinguish between base stations. In addition, the pilot signal may contain limited synchronization information, if the network is not synchronic.
It is advantageous from a practical point of view if the dispersion sequence of the pilot signal is directly in a known relation to synchronization bursts transmitted by the base station. Hereby base stations according to the CDMA system need not be in synchrony, but the mobile may calculate the timing of the adjacent station from the pilot signal which it receives from this.
The base station may also on a broadcasting channel transmit information to the cell that the pilot signal is in use, whereby mobiles able to utilize it may operate in a correct manner.
The mobile receives a carrier-frequency signal at other times than in the actual reception time slot, it decodes a pilot signal from it, if there is one, and it obtains information from the signal about the state of channels (time slots) in the uplink direction. When deciding to perform handover, it selects the free channel it desires and transmits an access burst on this to the base station. If the base station accepts the channel, it will transmit back an access grant burst on the same channel, whereupon the base station and the mobile will immediately transfer the traffic to this channel.